{"id":7929,"date":"2014-01-29T08:09:05","date_gmt":"2014-01-29T13:09:05","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7929"},"modified":"2026-04-17T21:03:07","modified_gmt":"2026-04-18T02:03:07","slug":"synthesis-5-reactions-of-alkynes","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/01\/29\/synthesis-5-reactions-of-alkynes\/","title":{"rendered":"Synthesis (5) – Reactions of Alkynes"},"content":{"rendered":"

A Reaction Map For Alkynes (PDF)<\/strong><\/p>\n

Today, we’re going to add the reactions of alkynes to our reaction map, which will bring to a close all the major reactions we’ve discussed so far in a typical first semester course.<\/p>\n

    \n
  1. Summary of Alkyne Reactions: Addition, Deprotonation (+ SN2), And Oxidative Cleavage<\/a><\/li>\n
  2. Key Reactions of Alkynes<\/a><\/li>\n
  3. Visualizing The Addition and Oxidative Cleavage Reactions of Alkynes With A Reaction Map<\/a><\/li>\n
  4. A Full Reaction Map PDF For First-Semester Organic Chemistry: Alkanes, Alkyl Halides, Alkenes, and Alkynes<\/a><\/li>\n
  5. Quiz Yourself!<\/a><\/li>\n
  6. Notes<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Summary of Alkyne Reactions: Addition, Deprotonation (+ SN2), And Oxidative Cleavage<\/h2>\n

    Like alkenes<\/a>, the main pathway found in the reactions of alkynes is “addition<\/strong>” – that is, breaking the C-C \u03c0\u00a0bond and forming two new single bonds to carbon. The product of an addition reaction to an alkyne is an alkene – and, as we just mentioned, alkene reactions undergo addition reactions too. The upshot of this is that since alkynes possess two \u03c0\u00a0bonds, one must always be alert to the possibility of two<\/strong> addition reactions occurring. Furthermore, should only one addition occur, the stereochemistry of the addition should be well noted, as it can lead to the formation of geometric isomers (see post: E and Z\u00a0 Notation for Alkenes)<\/a><\/em>.<\/p>\n

    Finally, there is an additional complexity in certain alkyne reactions that is not found in the reactions of alkenes. When water is added across an alkyne, the resulting product is an “enol”. (See post: Hydroboration and Oxymercuration of Alkynes<\/a><\/em>)<\/p>\n

    And enols, as you’ll learn more about in Org 2, tend to be fairly unstable species. Through a process called tautomerism, (See post: Keto-Enol Tautomerism<\/a><\/em>)they convert to their constitutional isomers containing carbonyl (C=O) groups such as aldehydes and ketones.<\/p>\n

    Another reaction not present in the reactions of alkenes is deprotonation<\/strong>. Alkynes are unusually acidic hydrocarbons, with a pKa<\/sub> of about 25 (compare that to alkenes (pka<\/sub> = 43) and alkanes (pKa<\/sub> = 50). The deprotonation of alkynes leads to its conjugate base, an “acetylide”, which is an excellent nucleophile (See post: Acetylides from Alkynes<\/a><\/em>)<\/p>\n

    The reaction of acetylides with alkyl halides (in SN<\/sub>2 reactions) is one of the few carbon-carbon bond forming reactions learned in Org 1, which makes it arguably the most important reaction to learn for synthesis this semester.<\/p>\n

    Finally, alkynes also undergo oxidative cleavage<\/strong> reactions. Treatment of alkynes with either ozone or KMnO4<\/sub> leads to carboxylic acids [terminal alkynes give carbonic acid, which decomposes to CO2<\/sub> and water<\/em><\/span>].<\/p>\n

    <\/a>2. Key Reactions of Alkynes<\/h2>\n

    Here’s a list of the key reaction types:<\/p>\n

    \"image<\/p>\n

    <\/a>3. Visualizing The Addition and Oxidative Cleavage Reactions of Alkynes With A Reaction Map<\/h2>\n

    One useful way to help visualize these reactions is to make a “spiderweb” diagram, showing how alkynes are transformed into a variety of functional groups. This is what it looks like for alkynes.<\/p>\n

    \"transformations<\/a><\/p>\n

    The previous post in the series on alkynes was entitled, “Alkynes Are A Blank Canvas<\/a>“. Alkynes are a blank canvas because on top of their own transformations, through partial reduction (Na\/NH3<\/sub><\/a> or Lindlar<\/a>)\u00a0 alkynes can also be transformed to alkenes, (which themselves have a host of reactions) or even alkanes (which can then be transformed to alkyl halides, which also have a host of reactions).<\/p>\n

    <\/a>4. A Full Reaction Map PDF For First-Semester Organic Chemistry: Alkanes, Alkyl Halides, Alkenes, and Alkynes<\/h2>\n

    This updated reaction map shows all the key reactions of alkanes, alkyl halides, alkenes, and alkynes covered in this and previous blog posts. One note – in a large map such as this, compromises had to be made: it is impossible to maintain complete self-consistency between all the structures drawn for each functional group and the resulting reactions. For example, on the sheet, alkynes are depicted as R-C\u2261C-R, (internal alkyne) while the products of certain reactions clearly come from a terminal alkyne. Each functional group should be interpreted figuratively (i.e. including its common variations) and not literally.<\/p>\n

    Test Yourself!<\/strong><\/p>\n

    See if you can use the map to find ways to do these transformations (in any number of steps):<\/p>\n